The present invention relates generally to semiconductor integrated circuits and methods for their fabrication and, more particularly, to aluminum interconnects and methods for their formation.
Semiconductor integrated circuits (ICs) contain individual devices, which are typically operatively-coupled together using metal lines. In most applications, the metal lines are formed on a different level than the devices, separated by an intermediate dielectric, such as silicon oxide or borophosphosilicate glass (BPSG). The most commonly used metal lines are aluminum. Interconnects are formed between individual devices and the metal lines. A typical interconnect between metal layers is composed of a via (i.e. opening) formed in an intermediate dielectric. Similarly, an interconnect between a metal layer and silicon is composed of a contact (i.e. opening) formed in an intermediate dielectric over an active device region. The via is filled with a metal, such as aluminum or tungsten. Aluminum has been preferred to date as an interconnect metal. Aluminum exhibits relatively low resistivity as compared to tungsten and, furthermore, is highly compatible with silicon oxide, which is often used as the insulative material surrounding a via. Furthermore, when metal lines are used, which are composed of aluminum, compatibility between the metal lines and the aluminum interconnect materials is optimized.
Interconnects often further contain a diffusion barrier layer sandwiched between the metal and the active device region at the bottom of the via. Such layers prevent intermixing of the metal and material from the active device region, such as silicon, which extends the life of the device. Passive titanium nitride diffusion barrier layers are the most common diffusion barrier layers. Such layers are typically formed over a refractory metal silicide layer. Titanium silicide is the most commonly used refractory metal silicide due to its relatively low resistivity. The use of titanium silicide between titanium nitride and the active device region is preferred due to its intermediate crystallographic characteristics between those of silicon and titanium nitride. The intermediate crystallographic characteristics prevent increased resistivity resulting from a contact solely between silicon and titanium nitride, whose crystallographic characteristics are very different.
Ideally, interconnects exhibit zero impedance to current flow, as exhibited in an ohmic contact (i.e. those which exhibit linear current vs. voltage characteristics), to provide optimum electrical performance. However, interconnects are not ideal and typically exhibit near linear characteristics at best.
One significant concern in depositing metal into a via is obtaining adequate step coverage of the via, particularly obtaining adequate step coverage is difficult when the vias have high aspect ratios (i.e. a large ratio of height to width of the via), as seen more often as IC densities increase. To mitigate this problem, chemical vapor deposition (CVD) is used to deposit the metal instead of physical vapor deposition (PVD). CVD is more apt to adequately fill high-aspect ratio contact holes than PVD. However, to date, CVD aluminum exhibits rough, nonconformal layers on complex topographies, prior to surface modification. This is undesirable because voids often develop within a via, due to the roughness of the CVD aluminum. Such voids severely increase the resistivity of a contact and degrade device performance by not providing uniform connection across an interconnect.
There is a need for an interconnect structure that effectively utilizes aluminum instead of tungsten. There is a further need for a method for forming a smooth, conformal aluminum layer within an interconnect structure. A method for using aluminum in sub-0.25 micron contact holes is needed in order to optimize future device performance.
The present invention provides a method and apparatus for fabricating an interconnect supported by a semiconductor structure. A first layer of titanium nitride is formed on the semiconductor structure. Then, a second layer of titanium nitride is formed on the first layer of titanium nitride. Finally, an aluminum film is formed on the second layer of titanium nitride. A titanium silicide layer is optionally formed on the semiconductor structure prior to the step of forming the first layer of titanium nitride.
In particular, according to one aspect of the method of the present invention, a first titanium nitride layer is formed on an active device region to act as a barrier layer, protecting the integrity of the contact. To accomplish this task, an amorphous titanium nitride layer is formed by reacting a titanium-containing precursor in the presence of nitrogen. Then, a second titanium nitride layer is formed on the first titanium nitride layer. The second titanium layer has a polycrystalline orientation (having a mixture of grains orientated in the  less than 111 greater than  and  less than 200 greater than  directions), which allows diffusion between the active device region and the interconnect metal. By forming a polycrystalline layer over the amorphous barrier layer, subsequent formation of small grain size chemical vapor deposition (CVD) aluminum is possible. The second titanium nitride layer is formed by reacting a titanium-containing precursor in the presence of at least ammonia (NH3) or nitrogen trifluoride (NF3). The titanium-containing precursors for forming the first and second layers of TiN are selected from the group consisting of: titanium tetrachloride, tetrakisdimethylamide titanium, and trimethylethylenediamino titanium. Finally, CVD aluminum is formed on the second titanium nitride layer to complete the interconnect. To form the CVD aluminum, an aluminum-containing precursor is used. The aluminum-containing precursor is selected from the group consisting of: trimethylaluminum (TMA), dimethylaluminum hydride (DMAH), triisobutylaluminum (TIBA), triethylaluminum (TEA), diethylaluminum hydride (DEAH), monomethylaluminum hydride (MMAH), dimethylethylalane (DMEHA1), and dimethylethylamide (DMEHA2).
In particular, according to another aspect of the method of the present invention, a titanium silicide layer is formed between the first titanium nitride layer and the underlying active device region to further improve the ohmic characteristics of the contact. Titanium silicide has an intermediate crystallographic structure between that of silicon in the active device region and titanium nitride. Thus, electronic carriers are able to diffuse easily through the interconnect structure.
The crystal structure and grain size of the underlayers are controlled to promote subsequent formation of a small grain size, conformal aluminum film in an interconnect. The aluminum film formed according to the method of the invention has a polycrystalline orientation, with grain sizes of less than approximately 0.25 microns. Thus, interconnects are formed with aluminum instead of tungsten, providing a lower resistivity contact to adjacent active device regions. Furthermore, such aluminum films are more conformal due to their smaller grain sizes. Vias are thus able to be filled with such aluminum to provide an interconnect structure substantially free of unwanted voids.